Thickness measuring method and apparatus employing microwaves



Jan. 7, 1964 J. B. BEYER ETAL 3,117,276

THIcxNEss MEASURING METHOD AND APPARATUS EMPLOYING MIcRowAvEs `4 sheets-sheet 1 Filed June 28', 1960 Jan. 7, 1964 J. B. BEYERV ETAL THICKNESS MEASURING METHOD AND APPARATUS EMPLOYING MICROWAVES Filed June 28, 1960 4 Sheets-Sheet 2 Jan. 7, 1964 THICKNESS MEASURING METHOD AND APPARATUS EMPLOYING MICROWAVES Filed June 28, 1960 4 Sheets-Sheet 3 Jan. 7, 1964 J. B. BEYER- ETAL THICKNESS MEASURING METHOD AND APPARATUS EMPLOYING MICROWAVES Filed June 28, 1960 4 sheets-sheet- 4 llllllllllllll- W K sin www T 4mm wm fw. Il I I i l I IIJ w 7 M 2 ,0 m w mi j ma 7 m a e M MWL/@ M n M W/ f W uw@ m m wv. e. rulli. ...IIIIL f T mw .ff fam 1% This invention relates to methods and apparatus for measuring the thickness of materials and more particularly to microwave circuits for continuously measuring and controlling the thickness of a moving material.

Various methods of measuring the thickness of materials, stationary or moving, have been employed in the past. rIhe problem in measuring the thickness of a material that is moving is typiiied by the control of a continuously moving sheet of steel strip in a rolling mill. In general, in both hot and cold rolling steel mills, a sheet of steel to be reduced in thickness is passed between a pair of rolls having a preselected separation related to the desired steel thickness. Therefore, the variations in the separation of the rolls from the preselected separation has a direct relationship on the thickness of the steel and the variations of the steel strip emerging therefrom. In the steel industry these measurements and controls have been eiiected in the past by means of measuring the amount of beta radiation absorption as an indication oi the thickness of a steel strip or the inclusion in a capacitance bridge of a capacitor formed by the material under measurement and a iixed capacitor plate. A very popular method of measurement in the steel industry involves the use of X-ray absorption. This latter method or" measurement depends on the measurement of absorption of the radiation and consequently the composition of the material undergoing measurement must be accurately known. This, then, places an important restriction on this method of measurement. in addition, bulky, expensive, and considerable electronic equipment for the detector and X-ray source is required.

This invention provides improved and simple thickness measuring methods and apparatus and which methods and apparatus are incorporated in a control system for continuously monitoring and maintaining the thic ess or" the material undergoing measurement within preseiected tolerances. This method of measure provides a compact, inexpensive system resulting in excellent sensitivity and rapid response. The improved method and apparatus of this invention measures the thickness of a materia possessing good reflecting properties at microwave frequencies and utilizes microwave techniques to effect the measurement without any contact of the material being measured and not dependent on the position of the material under measurement, or its composition, surface conditions, and translatory motion. When the methods of this invention are employed in a control system, the resulting system is advantageously characterized by excellent sensitivity and rapid response toy effect any correction indicated by the error measuring system.

This invention comprises separate means such as microwave antennas for radiating a microwave signal towards opposite sides of the material to be measured or a reflecting surface and for receiving the reflected signal therefrom.

The reflected signals are employed in separate microwave measuring systems for measuring the thickness of the material undergoing measurement when the distance between the antennas is known. These reected signals are separated fom their transmitting signals and applied to appropriate detectors to provide signals having information indicative of the distance between the adjacent antenna and the material undergoing test. This pair of signals may then be combined in a mixer or adder to produce a resultant signal that is a function of the sum of the distances between the antennas. This arrangement therefore `defines an improved measuring arrangement and gauge that may be utilized in various control systems.

ln one embodiment of the invention the incident signal is a modulated signal and the distance information is extrated from the amplitude ofthe reflected signal to provide the distance information. When the amplitude method is utilized and a crystal detector provides the distance information, a square root correction network may be utilized in combination with the crystal detectors to correct for the square law characteristic of the crystal and thereby allow an increased operating range.

ln another embodiment of the invention the linear operating range may be increased by utilizing superheterodyne techniques. ln this particular embodiment the incident signal is an unmodulated signal and is utilized in combination with the signal provided by a local oscillator to eliminate the square law characteristic of the crystal detector.

in still another embodiment of the invention the distance information may be derived from the reflected signals by detecting the phase difference of the two signals in lthe same general arrangement as utilized in the amplitude detection arrangement. Both or" the amplitude and phase detection arrangements may be employed in a feedback control or closed loop circuit to continuously measure and monitor a continuously advancing sample, as in a steel rolling mill.

These and other features of the present invention may be more fully appreciated when considered in the light of the following specification and drawings, in which:

FIG. 1 is a block diagram of a control arrangement embodying the invention;

FIGS. Z and 3 are perspective views of slot antennas and dielectric antennas respectively for use in a system of the type of FIG. l;

FIG. 4 is a schematic representation of the square root correction network for use in the system of FlG. l;

FIG. 5 is a graphic representation of the input versus output Voltage characteristic of the network of Fl'G. 4;

FIG. 6 is a schematic block diagram of a modilied dctector arrangement for use in the system of FIG. 1 utilizing the square root network of FIG. 4;

(FIG. 7 is a schematic block diagram of a supcrheterodyne control arrangement of the type of FIG. l;

FiG. 8 is a perspective view of a phase detector or discriminator for use in the system of FIG. l0;

FIF. 8a is a schematic representation of the phase detector arrangement of FiG. 8;

FIG. 9 is a graphic representation of a phasor diagram for the phase detector of FG. 8; and

FIG. l0 is a block diagram of a modified control arrangement operating on the phase relationship of the rciiected signals.

The basic `arrangement of Lhe invention for measuring or gauging the thickness of a sample of a material to be measured, such as the steel strip shown in FlG. 1, will now be examined. rfhe invention utilizes two independent microwave measuring systems for measuring the distance from fixed microwave antennas or feed horns to each side of the material to be measured. The material to be measured is subjected to the microwave radiation from the antennas whereby the energy is reected from opposite sides of the material and only the reflected radiation is applied to individual microwave measuring systems. As indicated in FiG. l, the incident waves V1 and V2 are related to the reflected waves by a complex reiection coemcient I1 and T2, respectively. Both T1 and T2 depend on the respective `distances dl and d2, in phase and amplitude. The distances d1 and d2 are the distances between the reflecting surface of the material and the antenna from which the incident radiation is derived. Therefore, the reflected signals LV1 and I2V2 have phase and `amplitude characteristics that contain information that is directly related to the distances d1 and d2. By applying the reected signals IlVl and IZVZ to an appropriate detector, the distance information contained therein may be extracted from the reflected signals to independently measure the distances d1 and d2.

The detectors utilized for this purpose must have a linear region in their transfer characteristic and the detectors must be operated over this linear portion in order that the output signal from the detector may truly represent the distances d1 and d2 with changes in thickness of the material undergoing measurement or test. This condition of operation is necessary since operation in the non-linear range will result in an output indica-tion not only vfor changes in thickness but `also for changes in position of the sample undergoing measurement. Assuming the thickness of the material undergoing measurement is represented by e, any changes in thickness, Ae, will change the distance dl-l-dz by A6, Therefore, by combining the quantities or signals representative of the distances d1 and d2 so that the resultant depends on the sum of dl and d2 only, this resultant will then provide a direct measure of the changes in thickness, e. This resultant quantity `will also be independent of any change in the position of the sample between the antennas since the sum of the distances dl-l-dg will remain a constant with the changes in position of the sample.

General considerations, therefore, lead to the conclusion that the thickness of a sample may be measured or, with -a known thickness, the relative `distances between the sample and the antennas may be determined by the double reflection of microwave signals from opposite surfaces of the sample. The desired information is represented by the phase and amplitude of the two reflected signals and, therefore, the desired information may be derived either by detecting the amplitude characteristics of the reflected signals, the phase relationship of the reflected signals, or a combination of both.

The measuring appara-tus of the present invention will now be described in more detail by reference to FIG. 1 wherein the invention is illustrated for detecting the amplitude characteristic of the reflected waves to extract the desired information and employing a modulated incident signal. rihe material under measurement, or the element having the necessary reflecting surfaces, is identified by the reference character 10. for the present, the material llt? will be assumed to be in a stationary position.

The material il@ receives the microwave energy'or radiation derived from a signal source l2, shown in dotted outline. The signal source l2 is coupled to a pair of microwave antennas 113 and 14 by means of individual separators 15 and lid. The reflected energy derived from the separators llS and lr6 is applied to the individual amplitude detectors 1l7 and 113. The output signals from the detectors i7 and 1? are, in turn, combined in a mixer Lid 4 1.19 providing the necessary distance or thickness information signal.

The material )lfb undergoing measurement or test is characterized as `a substantially perfect reecting surface and, therefore, must have a negligible penetration to the microwave energy. Most of the metallic surfaces, including the steel strips used in hot and cold rolling mills, provide this characteristic.

rlhe signal source i2 comprises a microwave signal generator shown as a klystron 2l. The klystron 2l provides a modulated output signal through the provision of a modulating signal source Z2 coupled to the repeller electrode of the klystron 21. The klystron 21 provides a signal on the order of 9,150 megacycles and is modulated by a 1,000 cycle pulse signal from the modulation signal source 22,. In order to iallow the detectors i7 and l to operate over their linear range, the frequency of the klystron 2l should be maintained within a few megacycles. This frequency stability is obtainable by a commercially available ZKZS reflux klystron having a standard regulated power supply. The modulated output signal from the vklystron 21 is coupled to a ferrite isolator 2S functioning to prevent frequency pulling of the klystron 2d by the reflected signal in the separators l5 and i6. The remaining element comprising the signal source l2 is a hybrid junction 24. The hybrid junction 24 is provided to effect isolation between the two independent microwave systems measuring the distances d1 and d2. The hybrid junction 24 is connected to be responsive to the output signal from the ferrite isolator 23 and delivers the mircowave energy to the separators 115' and 16. The microwave energy is coupled to the antennas i3 and 14 by means of the separators l5 and 16.

The 4antennas llf2` and i4 `are arranged on opposite sides of the material 1li* to direct or transmit the microwave energy to the adjacent reflecting surfaces thereof. The antennas 13 and 14 are of identical and conventional horn-type radiators. The characteristic for the antennas 13 rand 14 that is required, for the purposes of this invention, is `a linear portion in their .amplitude versus distance characteristic. The horn radiators should be capable of providing a narrow beam of radiated energy and recovering as much of the radiated energy as possible 4and to obtain a well defined variation of the reflected signal with distance. Several types of antennas are known to comply with these requirements and were described in 1948 in an article by R. B. Watson and C. W. Horton, appearing in Journal of Applied Physics, volume XIX, page 661. A slotted antenna similar to the one shown in FIG. 2 has been found to have a linear range of about 1.5 millimeters land its compact construction has been found to ,be satisfactory. Other antennas described in the above-identified publication possess slightly larger linear ranges but are bulkier. The dielectric or lucite `antenna illustrated in FlG. 3 is an example of these types of antennas.

The separators l5 and i6 each comprise a wave guide or slotted line 25 and 26 coupled between the antennas 113 and 14 and an individual directional coupler 2.7 and 28 respectively. The wave guides 25 and EJ couple the microwave energy, V1 and V2, from the signal source l2 to the respective antennas i3 und ld as received from their respective directional couplers 27 and 28. 'he reflected waves detected by the antennas y13 and le are, in turn, delivered by the respective wave guides 25 and 26 to their directional couplers 27 and 2d. The directional couplers are well-known in the art and the couplers 27 and 28 separate the reflected signal from the incident signal, or signal to be transmitted, and hence the only microwave enengy coupled to the detectors 17 and i8 is due to the reflected signal.

The detectors 17 and 13 may be standard microwave crystals, commercially identified as lNZSB crystals, and arranged in standard microwave crystal mounts. As is well-known, the crystal detectors operate on the square aunar/fl law characteristic and, therefore, the output signals thereof are non-linear. rthe detectors, however, do have a linear range and, as long as they are operated in this linear range, only the thickness changes, ne, |will be produced therefrom. This linear region has been determined to be on the order of two millimeters and has produced satisfatcory results without correction therefor. In addition, it should be noted that, in view of their square law property', the output signals that are produced from the detectors i7 and 18 are proportion-al to Athe square of the distances d1 and d2.

The mixer i9 receives the two distance signals from the detectors i7 and la? and combines them to produce a signal that is a function or" the sum of the distances d1 and d2. The mixer i9 may ltake many forms and a convenient errang-'ement for the purposes of this invention is a resistance adder having an adjustable leg. The resistance adder comprises the iii-:ed resistor R2 and the variable resistor R1 respectively responsive to the detectors 13 i7. The currents llowing in the resistors R1 and R2 are proportional to the distances d1 and d2 and, yaccordingly, the sum or output current will be proportional to Irl-7,152 wherein 17 is equal to rEhe value of R2 ymay be adjusted whereby equal changes in the distances d1 and d2 produce equal outputs.

lt should now be evident that the modulated microwave energy is radiated in a narrow pattern towards the material and reliccted th refrom and received by the s e antennas and, as a result of the separation of the signals provided by the directional couplers 27 and 28, the reflected signals I'lVl and P2K/2 are applied to the respective crystal detectors l? and lll. As long as the ldystron El is maintained lwithin its frequency limits the det ctcrs l and 1.3 are operated on the llat portions or their translation curve, an output signal which is a linear ft of the thickness e of the material -l'd will be provd Vto the mim: i?. The combining ol' this oi signals by the mixer l@ produces a resultant signal that is indicative of the thickness e oi theV material. The limitation `on the above-described device is the result of the square law characteristic of the detectors i7 and 18 and, if nnproved results are desirable or necessary, a diierent mode oi operation or correction for the square law characteristic of the detectors i7 and id is necessary.

To imzove the operation of the measuring apparatus by compensating for the s `are law characteristic of the detectors i7 and lS, a square root device may be incorporated into the measuring apparatus or Pi. l to be responsive to the signals from the detectors l? and l. A very simple square root device 39 is shown in FlG. 4 and its accompanying transfer function is shown in FiG. 5. The square root device Sil comprises a resistor 3l connected in series with a crystal rectifier 32. The resistance value of the resistor 3l is on the order of lltCC ohms and is much larger than the forward resistance of the 1N34A crystal 32. This resistance relationship causes the input current to the device to be linearly related to inout voltage. Under these conditions tl e output voltage becomes proportional to the square root of the input current because of `the crystals square law characteristic. Therefore, die output voltage Bout is proportional to the square root of the input voltage \/En. The operating range for the square root device 3l) may be seen by reference to Fl-G. 5 to be lore `than adequate for the ptn-'poses of this invention. Accordingly, by employing the square root device 39 in the arrangement of FIG. l, the overall amplitude versus distance characteristic of the detectors i7 and 1S become linear.

Now referring to FlG. 6, the portion of the system of FlG. l including the square law compensated arrangement will be described. The arrangement of FlG. 6 merely indicates the modified construction for the detectors 17 and LS to incorporate Ithe square root device fill therein. The detectors 17 and t8 employ the same crystals as described hereinabove to detect the rel'lected signal and which amplitude varying reflection signals are app-lied to resistance capacitance coupled ampliiiers 33 and 34. The amplifiers 33 and 3d may be :two-stage conventional RC coupled ampliers having their second stages coupled to the square root devices 30. It should now be apparent that `the voltages derived from the detectors ll7 and dS, in accordance with the arrangement of FlG. 6, are linearly related to the distances d1 and d2. These voltages are then applied to the mixer l) to produce .the output signal proportional to the sum ol the distances drt-d2. it has been found that the utilization of the square root device 3d provides an increase of approximately three times in the width of the lineal' range of the measuring apparatus as contrasted with the operation Iwithout the use of the square root devices 3d.

The undesirable elect of the square law characteristic of the crystal detectors i7 and 1S may be eliminated by employing an Aunmodulated incident signal along with the superheterodyne detection in an arrangement similar to the one shown in FIG. 7. Under this method of operation the signal source l2 comprises an unmodulated signal source 36 which may be the saine klystron as provided for the source 2l but, in this instance, the microwave energy is provided at 9,300 megacycles per second. The unmodulated microwave energy is applied to the directional couplers 27 and 2d as in the previous embodiments. A microwave signal of 9,330 megacycles per second derived from a local oscillator 31'? is applied to a hybrid, junction 24 to isolate the detectors l? and l from each other, while allowing them to each be responsive to the output signal provided by the local oscillator 37. The output signals from the detectors i7 and l5 .re the difference frequency, 30 megacycles, and are ap- Ylied to intermediate frequency amplifiers 3S and 39 arranged in combination with linear detectors. The direct current signals provided at the output circuits of the lin-ear detectors are combined in the resistance adder or mixer 19. Since the detection at the output of the 1F ampi crs 3S and 59 are at a high level, the desired linear amplitude versus distance characteristic is maintained'.

t should be noted that the main advantage of the superneterodyne method is the elimination of the square law characteristic of the crystal detectors since an increase in width of linear lrange of operation does not occur. It should also be note-d that the extent of the linear region of the amplitude versus distance characteristic is not a function of the square root device 3d and, therefore, the superhetercdyne method offers no advantage over the direct amplitude detection method. ln some instances it may be found to be disadvantageous to employ the superheterodyne technique because of the noise `added by the ampliliers and the drift of the two klystrons 3o and 37.

Now returning to FIG. l the amplitude detecting arrangement as applied to a control system for continuously measuring and monitoring the reduction rolls for a sheet or" strip steel will be described. The strip steel is identilied by the reference character d@ and is continuously advanced through Aa pair of rolls l5 and de spaced apart a distance cor-responding to the desired thickness for the material or steel strip llt'. Alt should be noted that although the description will continue with regard to the direct amplitude `detection method, the same system is applicable to the arrangement utilizing the square root correction network and the superheterodyne method dcrscribed hereinabove.

When the system of FIG. l is utilized to control the thickness of the material l@ and thereby the spacing of the rolls 45 and 46m, it is desired to space the microwave antennas as close as possible to the point of contact between the material lill and the rolls d5 and 4l-6. To this end7 the compactness of the slot antenna of FIG. 2 allows it to be spaced close to the rolls [l5 and do. This, therefore, minimizes the transport time lag and sheet translation errors. To provide a feedback control signal, the output signal derived from the mixer or the resistance adder l@ proportional to the `distances d1 and d2 is applied to a difference ampliiier or comparison device i7 The difference ampliiier 47 is also connected to be responsive to a reference signal provided by the reference source shown in block form and identified by the reference character d. rlhe reference signal provided by the source is representative of the desired thickness e for the material lil. lt will be recognized that although the reference signal source i8 is shown as a separate block, it may be derived directly `from the klystron 2l. When the reference signal is provided in this fashion, the effects of unwanted amplitude variations in the klystron output lare 'educed and also provides a zero output signal for some desired nominal thickness for the strip litt. The magnitude of the nominal thickness is unimportant as long as it does not approach the skin depth.

The difference or error signal provided by the amplifier :7i is applied to a roll control circuit shown in block form and identified by the reference character '49. The roll control circuit 49 functions to change the position of the rolls in accordance with the error signal to the position of the rolls d5 and do to thereby produce the desire-d thickness for the sheet steel lil.

it should now be apparent from the above description that, as the material lll* is continuously advanced through the rolls i5 and do, the thickness of the material is continually measured and the position of the rolls adjusted, when necessary, to correct for any variations in thickness from the desired thickness. The sheet steel is radiated with the microwave energy from the antennas ll3 and lll and the pair of reilected waves `from the opposite surfaces of the steel strip lil are coupled to the detectors @7 and 'i3 to produce the independent distance signals proportional to the distances d1 and d2. The distance signals are then combined in the mixer i9 to produce a signal that is the function of (dri-d2) and compared in the difference amplifer 47 with the signal representative of the desired thickness. Any measured deviation lfrom the desired thickness will cause an error signal or control signal to be applied to the roll control circuit i9 to provide the adjustment for the rolls [l5 Iand l5 and thereby continuously maintain the steel strip lil at the desired thickness. ideally, when the measured thickness and the desired thickness are equivalent, the difference amplifier 47 will provide a zero signal. To this end, the above system g has been used wherein a constant thickness sheet traveling between dielectric antennas at one-half meter per second gave no output variation, while thickness changes of onefortieth of a millimeter were clearly detected. lt was further found that both the composition and the surface conditions of the material undergoing measurement were relatively unimportant and that translations of the sample by 1.5 millimeters or less yielded no change in output.

The reflected signals IlVl and Tfr/2 were described hereinabove las containing the desired distance information and that either the phase or the amplitude of these signals may be independently detected in order to provide the desired output indication. By utilizing a system analogous in principal to the amplitude detecting systems described hereinabove, the distances d1 and d2 may be independently measured by means of the phase relationships of the two reflected signals. The reflected waves vare applied to separate phase detectors to extract the desired phase information and the resulting output signals are then applied to a mixer or adder circuit as described for the amplitude detection arrangement. The phase detectors, ideally, should be amplitude insensitive.

A phase sensitive detector or phase discriminator for the purposes of this invention is more complex than the microwave crystal amplitude detector. A phase detector 5l. which has been successfully employed is a variation 'or the hybrid junction detector. The phase detector Sil is shown in FIG. 8 and the schematic representation thereof is shown in HG. 8a. The phase detector 5l comprises pair of arms arranged at right angles to one another, the vertical arm is identiied as the E plane arm, while the horieontal arm is identified as the H plane arm. Between the E and H plane arms and arranged in the same horizontal plane as the H plane arm and in a 90 degree relationship with each ot the E and H plane arms are a pair of side arms, identified as side arm one and two, shown respectively to the left and right of the E plane arm.

The Aoperation of the phase detector 5l will now bc described, assuming a reference ase signal B is applied to the H plane arm and it is desired to measure the changes in the phase of a varialle signal A. The signal A is applied to the l5 piane arm and is assumed to 'ce adjusted to an angle ot 9) degrees with respect to the signal and the side arm one and thereby assumes an angle of (904480) degrees with respect to the signal B in side arm two because of the properties of hy" rid junction. rl`he phaser diagram shown in F153. 9 graphically illustrates the assumed relationship oi the signals A and i3 and the magnitude of the phasors (A -l-L) and (-A-l-B) and that these latter two signals are equal in magnitude. Therefore, when the phasors have equal magnitudes, the crystal detectors to which these signals are applied will have equal output signals, assuming the crystal characteristics to be identical. lt should he noted that the magnitudes of these two output signals will remain equal to each other reg dless of the changes in magnitudes or" the signals A and B, as long as the assumed 90 degree phase angle relationship is maintained between the signals. A comparison ot these two output signals (A -l-B) and (-/l -l-B), in a balanced diiierencc amplier will produce a zero output signal. if the phase angle of the signal A decreases, a reference to the i, diavram of FIG. 9 indicates that the magnitude of the signal of (A +B) will increase, while that oi the correspending signal (-A-I-B) decreases. Upon comparison in a balanced difference amplifier, an output signal or" a given polarity will be produced indicative of the decrease of phase angle of the signal A. lf the phase of the signal A increases rather than decreases, the magnitude of the combination (A-l-B) decreases, while that or the combination (-A-l-B) increases and, again, a comparison of of these signals in a difference amplifier produces an output which will be of the opposite polarity from the output signal produced when the phase angle or" the signal A decreases and thereby indicating the increase angle A. Therefore, it is apparent that the phase detector or phase discriminator 5l detects and indicates not only the magnitude but also the direction of the phase si itt between a pair of applied signals.

The above-described phase detector 5l is incoreorated into the system of FlG. l0 for controlling the thickness of a strip of sheet steel l@ in the same general fashion as described for the system of FlG. l. "f3 and l/l utilized in this phase detection system arra ement are pref rably the dielectric antennas illustrated in FlG. 3. The antennas l and ld are coupled to the separators l5 and llo and which separators are provided with a signal from a signal source l2. The microwave energy from the signal source l2 functions as the reference signal and is also coupled to the phase detector 5l, shown in the same schematic form as represented in PEG. 8s. rlhe E plane arm in this instan-ce is provided a i reflecting termination whereby the reference signal derived from the side arms ne two, S-l and The two reflected signals, A and -A, are coupled to the crystals ..52 and 53 respectively by means of hybrirl junctions (t'or purposes of isolation) as shown. The phase of the reflected signal A derived from the antenna l is adg'usted to be 90 degrees leading with resp-ect to the rd LAS nhasor wav ence signal B as they arrive at crystal 52 while the phase of A is adjusted to be (904-180 degrees) leading with respect to signal B at crystal 53. The phase relationships of the reference and reflected signals, therefore, are the same as described hereinabove. ln the above description the signals A and A were the same signal, while in the present discussion they are independent signals. The phase diagram of PEG. 9, however, still applies. To obtain these desired phase relationships for the reflected signals A and A, a pair of adjustable phase Shifters 54 and 5S are coupled to the respective separators l5 and ld.

The output signals from the crystals 52 and 53 are coupled to appropriate amplifiers 55 and S7, respectively, and the amplified output signals coupled to the difference amplifier i7 providing the difference or error signal upon comparison. The amplifier i7 is a balanced difference amplifier. The difference signal provided by the amplifier i7 is applied to a differencer or subtraction circuit 53. The differencer 5S is also provided with a reference signal representative of the desired thickness for the material lf?. This reference signal is of the opposite polarity from the error signal and the resulting signal from the differencer 53 provides the control signal for the control circuit 49.

rThe operation of the system of FlG. l0 will now be described. lf a translation of the steel strip l@ takes place in either direction, the phasors A and A, as shown in FlG. 9, will always rotate in opposite directions and by an equal amount to cause the magnitudes of (A -l-B) and (-/i +B) to remain equal. Under these conditions the difference amplifier will ideally provide an output which is equal to zero. lf the thickness e of the material i6 changes, however, phaser A -A rotate in the same direction resulting in difference magnitudes for (A+B) and {-A +B), and thereby, a difference or error signal is applied to the differencer 5S. The dif-Terencer 53 compares the error signal with the thickness reference signal and provides a control signal representative of the difference in thickness between the measured thickness and the desired thickness of the strip Il@ The control signal is applied to the roll control circuit 49 to actuate the rolls in a direction that will tend to return the material to the reference thickness.

lt should also be recognized that the combination of the phase amplitude methods may be employed in accordance with the above invention. The advantage of combining the two methods is the elimination of the directi il couplers and 23, utilized in the amplitude method, othe ise it appears to be inferior to the amplitude method because of a reduction of about onehalf in the linear operating range. Also, although the invention has been principally described in systems for measuring thickness, the concept of this invention is applicable to measuring distances by reflecting the microwaves from the opposite of a reflecting surface ccntrally located between the two poins to be measured. This method of operation may be utilized to measure the length of a bridge or changes thereof.

What is claimed is:

l. A measuring d vice including, in combination, first means for transmit g a signal towards one side of a reflecting surface and for receiving the signal reflected therefrom, second means for transmitting a signal towards the opposite side of the refiecting surface and for receiving the signal reflected from said opposite side, first separation means coupled to said first means for separating a signal to be transmitted from the signal reflected from sai one side and providing an output signal corresponding to the reflected signal, second separation means coupled t0 said second means for separating a signal to be transmitted from the signal reflected from said opposite side and providing an output signal corresponding to the reflected signal, a source of signals to be transmitted coupled to the "rst and second separation means, first detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal from said first separation means over the said linear region of its transfer characteristics and providing an output signal proportional to the distance between said first means and the one side of the reflecting surface of the material to be measured, second detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal from said second separation means over the said linear region of its transfer characteristics and providing an output signal proportional to the distance between said second means and said opposite side of the reflecting surface, and means connected to be responsive to the output signals from the first and second detector means for combining `the applied signals.

2. A measuring device as dened in claim 1 wherein the first and second detector means have a square law characteristic.

3. A measuring device including, in combination, first antenna means for transmitting a microwave signal towards one side of a reflecting surface and for receiving the reflected microwave signal therefrom, second antenna means for transmitting a microwave signal towards the Opposite side of the reflecting surface and for receiving the refiected microwave signal from said opposite side, first separation means coupled to said first antenna means for separating a microwave signal to be transmitted from the microwave signal reflected from said one side and providing an output signal corresponding to the reflected signal, second separation means coupled to said second antenna means for separating a microwave signal to be transmitted from the signal reflected from said opposite side and providing an output signal corresponding to the reflected signal, a source of microwave signals to be transmitted from the first and second antenna means coupled to the first and second separation means, first microwave crystal detector means including a square root network coupled to be responsive to the output signal from said first separation means and providing an output signal proportional to the distance between said first antenna means and the one side of the reflecting surface, second microwave crystal detector means including a square root network coupled to be responsive to the output signal from said second separation means and providing an output signal proportional to the distance between said second means and said opposite side of the reflecting surface, and combining mems connected to be responsive to the output signals from the first and second detector means for providing a signal related to the distance between the first and second antenna means.

4. A measuring device including, in combination, first means for transmitting a signal towards one side of a material to be measured and for receiving the transmitted sigrial refiected therefrom, second means for transmitting a signal towards the opposite side of the material to be measured and for receiving the transmitted signal reflected from said opposite side, first separation means coupled to said first means for separating a signal to be transmitted from the signal reflected from said one side and providing an output signal corresponding to the reflected signal, second separation means coupled to said second means for separating a signal to be transmitted from the signal reflected from said opposite side and providing an output signal corresponding to the reflected signal, a source of signals to be transmitted coupled to the first and second se aiation means, first detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal from said first separation means over the said linear region of its transfer characteristics and providing an output signal proportional to the distance between said first means and the one side of the refiecting surface of the material to be measured, second detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal from said second separation means over the said linear region of its transfer characteristics and providing an output signal 11 proportional to the distance between said second means and said opposite reflecting surface of the material to be measured, a source of reference signals, and means connected to be responsive to the output signals from the first and second detector means and said reference signals for combining and comparing the applied signals.

5. A measuring device including, in combination, first means for transmitting an electrical signal towards one side of a material to be measured and for receiving the refiected signal therefrom, second means for transmitting an electrical signal towards the opposite side of a material to be measured and for receiving the signal reflected from said opposite side, first separation means coupled to said first means for separating a signal to be transmitted from the signal reiiected from said one side and providing an electrical output signal correspond-ing to the reflected signal, second separation means coupled to said second means for separating a signal to be transmitted from the signal reflected from said opposite side and providing an electrical output sign-al corresponding to the reflected signal, a source of electrical signals to be transmitted coupled to the first `and second separation means, rst amplitude detector means having a linear region in its transfer characteristics coupled to lbe responsive to the amplitude of the signal from said first separation means over the said linear region of its transfer characteristics 'and providing an electrical output signal proportional 'to the distance between said `first means and the one side 'of the reflectinor surface of the material to be measured, second amplitude detector means having a linear region in its transfer characteristics coupled to be responsive to the amplitude of the signal from said second separation 'means over the said linear region of its transfer character- Iistios 'and providing an output signal proportional to the distance betwen said second means and said opposite side of the 'reflecting surface of the Imaterial to be measured, adding means connected to be responsive to the touput signals `from the first and second detector means for providing a signal lrelated to the total distance traveled by the pair of reflected signals.

6. A measuring device as defined in claim 5 wherein said source of electrical signals are modulated signals.

7. A measuring device as defined in claim 6 wherein said first and second detector means include crystal detectors and a square root network.

8. A measuring device including, in combination, first means for transmitting an electrical signal towards one side of a material to be measured and for receiving the reflected signal therefrom, second means for transmitting an electrical signal towards the opposite side of a mate- .rial to be measured and for receiving the signal reflected from said opposite side, first separation means coupled to said first means for separating a signal to be transmitted from the signal reflected from said one side and providing an output signal corresponding to the reflected signal, second separation means coupled to said second means for separating a signal to be transmitted from the signal reflected `from said opposite side and providing an output signal corresponding to the reflected signal, first phase shifting means responsive to said first separation means and providing an output signal having a preselected phase shift, second phase shifting means responsive to `said second separator' means and providing an output vsignal having a preselected phase shift related to the lphase s' ift produced by said first phase shifting means, `first phase detector means having a linear region in its .transfer characteristics coupled to be responsive to the output signal from said first phase shifting means and the .signal to be transmitted providing an output signal proportional to the distance between said first means and 'the one side of the reflecting surface of the material to Ybe measured, second phase detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal from said second phase shifting means over `the ysaid linearnegion of its transfer characteristics and the signal to be transmitted providing an output signal proportional to the distance between said second means and said opposite retiectng surface of the material to be measured, a source of signals to be transmitted coupled to the first and second separation means and the first and second phase detector means, and means connected to be responsive to the output signals from the first and second phase detector means for providing a signal related to the total distance traveled by the pair of reflected signals.

9. A thickness measuring device including, in combination, means for continuously advancing a material to be measured, rst antenna means for transmitting a microwave signal towards one side of the advancing material to be measured and for receiving the reflected signal therefrom, second antenna means for transmitting a microwave signal towards the opposite side of the advancing material to be measured and for receiving the signal reflected from said opposite side, said first and second antenna means being spaced apart substantially the same distance from the opposite sides of the advancing material, first separation `means coupled to said first antenna for separating a signal to be transmitted from the signa-l Ircflected from said one side and providing an output signal corresponding to the reflected signal, second separation means coupled to said second antenna means for separating a signal to be transmitted from the signal reiiected from said opposite side and providing an output signal corresponding to the reflected signal, a source of microwave signals to be transmitted from the first and second antenna means coupled to the first and second separation means, first detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal `from said first separation means over the said linear region of its transfer characteristics and providing an output signa-l proportional to the distance between said first antenna means and the one side of the rciiecting surface of the advancing material, second detector means having a linear region in its transfer characteristics coupled to be responsive to the output signal from said second separation means over the said linear region or its transfer characteristics and providing an output signal proportional to the distance between said second antenna means and said opposite reflecting surface of the advancing material, a source of reference signals representative of a preselected thickness for the advancing material, means connected to be responsive to the `output signs from the first and second detector means and said reference signals for combining and comparing the applied signals and providing ya measurement signal therefrom related to the reference signal.

10. A thickness measuring device as dened in claim 9 wherein said means for advancing the material to be measured is adapted to control the thickness of the material, and includes means responsive to the measurement signal for continuously controlling the thickness of the advancing material.

ll. A thickness measuring device as defined in claim 9 wherein said first and second detector means are amplitude responsive detectors.

l2. A thickness measuring evice as deiiued in claim 9 wherein said first and second detector means are phase sensitive detectors.

13. A method of continuously measuring the thickness of a continuously moving material independent of translatory motion and position of the material being measured, including the steps of advancing a material to be measured, continuously reflecting microwave signais from opposite sides of the material to be measured, detecting the reflected signals and providing an electrical output indication representative of the distance traveled by the reliected signals to the advancing materials, and combining the electrical output indications and providing a composite signal proportional to the total distance traveled by the reflecting signals.

14. A method of continuously measuring the thickness of a continuously moving material independent of transiatory motion and position of the material being measured, including the steps or" arranging a pair of microwave antennas a preselected distance apart, continuously advancing a material to be measured between the antennas, substantially simultaneously transmitting a microwave signal from each antenna towards the material to be measured and receiving the reilected signals therefrom, detecting the reected signals and providing an electrical output indication representative of the distance traveled by the reilected signals from their respective antennas to the advancing materials, combining the electrical output indications and providing a composite signal proportional to the total distance traveled by the reflected signals, and comparing the composite signal with a signal representative of a desired thickness and providing a signal representative of their difference.

14 References Cited in the file of this patent UNITED STATES PATENTS 2,489,092 Larrick Nov. 22, 1949 2,576,772 Bernet et al Nov. 27, 1951 2,640,190 Rines May 26, 1953 2,952,296 Kofoid Sept. 13, 196() FOREIGN PATENTS 1,165,687 France Oct. 28, 1958 615,667 Great Britain Jan, 10, 1949 337,341 Switzerland May 15, 1959 OTHER REFERENCES Ryan et al.: Microwaves Used to Gbserve Commutator and Slip Ring Surfaces During Operation, Electrical Engineering, March 1954; pp. 251-255. 

1. A MEASURING DEVICE INCLUDING, IN COMBINATION, FIRST MEANS FOR TRANSMITTING A SIGNAL TOWARDS ONE SIDE OF A REFLECTING SURFACE AND FOR RECEIVING THE SIGNAL REFLECTED THEREFROM, SECOND MEANS FOR TRANSMITTING A SIGNAL TOWARDS THE OPPOSITE SIDE OF THE REFLECTING SURFACE AND FOR RECEIVING THE SIGNAL REFLECTED FROM SAID OPPOSITE SIDE, FIRST SEPARATION MEANS COUPLED TO SAID FIRST MEANS FOR SEPARATING A SIGNAL TO BE TRANSMITTED FROM THE SIGNAL REFLECTED FROM SAID ONE SIDE AND PROVIDING AN OUTPUT SIGNAL CORRESPONDING TO THE REFLECTED SIGNAL, SECOND SEPARATION MEANS COUPLED TO SAID SECOND MEANS FOR SEPARATING A SIGNAL TO BE TRANSMITTED FROM THE SIGNAL REFLECTED FROM SAID OPPOSITE SIDE AND PROVIDING AN OUTPUT SIGNAL CORRESPONDING TO THE REFLECTED SIGNAL, A SOURCE OF SIGNALS TO BE TRANSMITTED COUPLED TO THE FIRST AND SECOND SEPARATION MEANS, FIRST DETECTOR MEANS HAVING A LINEAR REGION IN ITS TRANSFER CHARACTERISTICS COUPLED TO BE RESPONSIVE TO THE OUTPUT SIGNAL FROM SAID FIRST SEPARATION MEANS OVER THE SAID LINEAR REGION OF ITS TRANSFER CHARACTERISTICS AND PROVIDING AN OUTPUT SIGNAL PROPORTIONAL TO THE DISTANCE BETWEEN SAID FIRST MEANS AND THE ONE SIDE OF THE REFLECTING SURFACE OF THE MATERIAL TO BE MEASURED, SECOND DETECTOR MEANS HAVING A LINEAR REGION IN ITS TRANSFER CHARACTERISTICS COUPLED TO BE RESPONSIVE TO THE OUTPUT SIGNAL FROM SAID SECOND SEPARATION MEANS OVER THE SAID LINEAR REGION OF ITS TRANSFER CHARACTERISTICS AND PROVIDING AN OUTPUT SIGNAL PROPORTIONAL TO THE DISTANCE BETWEEN SAID SECOND MEANS AND SAID OPPOSITE SIDE OF THE REFLECTING SURFACE, AND MEANS CONNECTED TO BE RESPONSIVE TO THE OUTPUT SIGNALS FROM THE FIRST AND SECOND DETECTOR MEANS FOR COMBINING THE APPLIED SIGNALS.
 13. A METHOD OF CONTINUOUSLY MEASURING THE THICKNESS OF A CONTINUOUSLY MOVING MATERIAL INDEPENDENT OF TRANSLATORY MOTION AND POSITION OF THE MATERIAL BEING MEASURED, INCLUDING THE STEPS OF ADVANCING A MATERIAL TO BE MEASURED, CONTINUOUSLY REFLECTING MICROWAVE SIGNALS FROM OPPOSITE SIDES OF THE MATERIAL TO BE MEASURED, DETECTING THE REFLECTED SIGNALS AND PROVIDING AN ELECTRICAL OUTPUT INDICATION REPRESENTATIVE OF THE DISTANCE TRAVELED BY THE REFLECTED SIGNALS TO THE ADVANCING MATERIALS, AND COMBINING THE ELECTRICAL OUTPUT INDICATIONS AND PROVIDING A COMPOSITE SIGNAL PROPORTIONAL TO THE TOTAL DISTANCE TRAVELED BY THE REFLECTING SIGNALS. 